1. Field of the Invention:
This invention relates to turbo-charged engine systems.
2. Description of the Prior Art:
Generally, a well-known turbo-charger engine system has a single turbo-charger designed so as to exhibit the maximum torque at all times at engine speeds required for the engine. Therefore, the charging pressure ratio produced is low at low speeds as shown by the dashed line in FIG. 5. Therefore, the torque produced by the engine is low at low speeds, as shown by solid lines in the same Figure. The energy of exhaust gas is insufficient at an idle engine speed, the turbo-charger speed is low and cannot follow a sudden acceleration of the engine, and a so-called turbo-lag occurs.
In an attempt to achieve a torque curve as shown by the chain dotted line in FIG. 5 in which the low-speed torque is increased and the turbo-lag is decreased, there has been proposed a sequential turbo-charger engine system which includes a plurality of 2-chargers including a primary turbo-charger which produces a torque required at low speeds and a secondary turbo-charger which produces a torque supplementing the former torque at high speeds in which turbo-chargers are selectively operated in accordance with engine speed to thereby increase the charging pressure at low speeds and hence the low-speed torque.
During acceleration, when the number of turbo-chargers operated in the above system is increased, the exhaust switching valve is suddenly opened and the secondary turbo-chargers which have previously been at rest are operated. Therefore, the turbine output of the primary turbo-charger decreases due to a sudden increase in the turbine nozzle area and a retardation in the acceleration of the second turbo-chargers causes a temporary decrease in the charging pressure. Thus the charging pressure is lowered to thereby decrease the torque as shown by the solid line in FIG. 6, and the control of the engine output required by the vehicle, shown by a chain-dotted line in the same Figure, becomes temporarily impossible.